Try this Michelle, it may be that the XP built in firewall needs to allow it :
1. Click Start, click Run, type gpedit.msc, and then click OK.
2. Under Console Root, expand Computer Configuration, expand Administrative Templates, expand Network, expand Network Connections, expand Windows Firewall, and then click Domain Profile.
3. Right-click Windows Firewall: Allow remote administration exception, and then click Properties.
4. Click Enabled, and then click OK.
Also check that if you are using another firewall that its not that stopping it too, try temporarily disabling it and see if it works